This fixes the cachix interaction for PRs from forks. Previously the builds would fail because the cachix auth token would be missing. This change allows the token to be missing if the PR is from a fork but still requires it for in-repo PRs (so the cache will still get updated by in-repo builds).
This fixes the cachix interaction for PRs from forks. Previously the builds would fail because the cachix auth token would be missing. This change allows the token to be missing if the PR is from a fork but still requires it for in-repo PRs (so the cache will still get updated by in-repo builds).